For the Rub:Mix salt, pepper, brown sugar, garlic powder, onion powder, smoked paprika, hot paprika, and mustard powder together in a small bowl. Coat brisket liberally with rub and let sit for at least 40 minutes and up to a day, refrigerated.
Combine all the sauce ingredients together in the bowl of a food processor and pulse until combined.Set aside.
Preheat oven to 325°F. Heat oil in a large Dutch oven over high heat until shimmering. Place brisket in Dutch oven and cook until well browned, 6 to 8 minutes. Flip and brown on second side, about 5 minutes longer. I like to brown the sides of the brisket as well. Every single one! Transfer brisket to a large plate and lower heat to medium.
Add onion, garlic, to Dutch oven. Cook, stirring occasionally, until onions are softened, about 10 minutes. Add cranberry sauce mixture and scrape up browned bits from bottom of pan. Add beef broth. Stir to combine. Bring sauce to a boil. Nestle brisket in sauce, pouring in any accumulated juices from the plate. Cover Dutch oven and transfer to oven. Cook until brisket can be pierced with a fork with little to no resistance, about 3 hours.
ransfer brisket to a large container and add sauce. Cover and place in refrigerator overnight and up to three days (see note).
To Serve:Preheat oven to 325°F. Spoon off any congealed fat from sauce. Remove brisket from sauce and cut into 1/4-inch slices against the grain. Place slices in a casserole dish and cover with sauce. Cover dish with foil and place in oven until brisket is warmed through, 30 to 45 minutes. Serve immediately.